1. Field of the Invention
The present invention relates to a signal read technique in an image capturing apparatus.
2. Description of the Related Art
FIG. 1 is a circuit block diagram for explaining the configurations of an image sensing signal processing system and sensitivity/exposure control system of a conventional digital still camera using a solid state image sensor.
Reference numeral 101 denotes a lens for forming an object image on an image surface; 102, a stop for controlling the amount of light that has passed through the lens; 103, a mechanical shutter for allowing light which has passed through the lens to enter the image surface for a required period of time; and 104, a solid state image sensor which performs photo-electric conversion for the formed object image. Note that a main example of the solid state image sensor 104 is a CCD area sensor.
In recent years, an X-Y address type CMOS area sensor has been increasingly used as an image sensor. Hence, an image capturing apparatus using the CMOS area sensor will be described below.
Reference numeral 105 denotes an image sensor drive circuit which supplies a pulse having an amplitude for driving the CMOS area sensor; 106, a CDS circuit for performing correlated double sampling for the output from the CMOS area sensor; and 107, an AGC circuit for amplifying the output signal from the CDS circuit. The gain setting of the AGC circuit changes when a user changes the sensitivity setting of a camera depending on preference, or when the camera automatically increases the gain at a low luminance.
Reference numeral 108 denotes a clamp circuit for clamping an OB (optical black) potential (to be described later) to a reference potential of the output signal from the AGC circuit; and 109, an A/D conversion circuit for converting an analog image sensing signal output from the clamp circuit into a digital signal.
Reference numeral 110 denotes a video processing circuit having a video signal processing circuit 111 which processes the converted digital image sensing signal to luminance and color video signals (color difference signals such as R-Y and B-Y, or R, G, and B signals). The video processing circuit 110 also has a photometry circuit 112 which performs photometry on the basis of the level of a signal input from the CMOS area sensor. The video processing circuit 110 also has a WB (white balance) circuit or the like (not shown) which measures the color temperature of the object based on the signal input from the CMOS area sensor to extract information for performing white balance in the video signal processing circuit 111.
Reference numeral 113 denotes a timing pulse generation circuit which generates a timing pulse for each circuit in the camera; and 114, a CPU for controlling the camera. The CPU 114 has a function of instructing to change the gain of the above-described AGC circuit 107, and instructing an exposure control circuit 116 how to perform exposure, in order to control sensitivity and exposure based on the information from the above-described photometry circuit 112.
FIG. 2 is a circuit diagram showing the conventional image capturing apparatus using the CMOS area sensor.
Unit cells are arrayed in a two-dimensional matrix, each of which has an amplification transistor 202 which amplifies a detection signal from a photodiode 201, a vertical selection transistor 203 which selects a line from which a signal is to be read, and a reset transistor 204 which resets signal charge. Note that the cells are arrayed in a 3×3 matrix in FIG. 2. However, more unit cells are actually arrayed.
A horizontal address line 206 is horizontally connected from a vertical shift register 205 to the gate of the vertical selection transistor 203 to select a line from which a signal is to be read. Likewise, a reset line 207 is horizontally connected from the vertical shift register 205 to the gate of the reset transistor 204. The source of the amplification transistor 202 is connected to a vertical signal line 208 connected in the column direction, and a load transistor 209 is connected to one end of the vertical signal line 208. The other end of the vertical signal line 208 is connected to a horizontal signal line 211 via a horizontal selection transistor 212 driven in accordance with a selection pulse supplied from a horizontal shift register 210.
As shown in FIG. 3, the solid state image sensor has an effective image sensing pixel area 301 and an OB (optical black) pixel area 302 for adjusting an optical black reference. In the OB pixel area 302, photo-electric conversion elements (pixels) arranged in the outer frame portion of the solid state image sensor are shielded from light by aluminum or the like. The output from the light-shielded area is used as the optical black reference to perform video adjustment, i.e., so-called a clamping process.
In moving image photographing in such conventional image capturing apparatus, electric charge accumulated in each photo-electric conversion element is read once per frame period, and one image is contained in one frame of the sensed video. In this case, as shown in FIG. 4A, the output signal in the OB pixel area is read once in a 1H (horizontal synchronization) period.
In a given image capturing apparatus, a stable OB clamping operation is executed by reading the same position in the OB pixel area regardless of the read modes of an arbitrary area and pixel signals, thereby preferably suppressing an FPN (fixed pattern noise) (see Japanese Patent Laid-Open No. 9-366193).
Furthermore, Japanese Patent Laid-Open No. 9-163236 discloses an image capturing apparatus which uses an X-Y address type solid state image sensor having the OB pixel area shielded from light in the outer frame portion of the effective image sensing area, and can be driven in a mode of reading pixels once every arbitrary number of pixels, or reading all pixels in the effective image sensing area. In this method, the OB pixel area may be read at a low rate by thinning out or decimating the pixels in the OB pixel area when the pixels in the effective image sensing area are read once every arbitrary number of pixels in a thinning-out mode, or may be read without thinning out the pixels in the OB pixel area even when the pixels in the effective image sensing area are read once every arbitrary number of pixels in the thinning-out mode.
In recent years, a frame rate is taken into consideration which especially influences the characteristics of a moving image photographing operation. Market needs for a high-resolution moving image whose size is VGA (640 (horizontal)×480 (vertical) pixels) or more in the image capturing apparatus such as a digital camera or digital camcorder.
A larger number of pixel signals must be read for each frame of the image sensor in order to photograph a high-resolution moving image. Accordingly, it is difficult to photograph a moving image at a high frame rate.
As general drive methods of reading the entire frame in the same mode, a thinning-out mode of thinning out pixels to ½ as shown in FIG. 4B, and a thinning-out mode of thinning out the pixels to ¼ as shown in FIG. 4C are available. The image capturing apparatus having these read modes must set the OB pixel area so as to ensure a sufficient OB clamp time at the highest thinning-out rate. Note that “OB clamping” represents an operation of subtracting signals in the OB pixel area from those in the effective image sensing area to remove, from the image signals, dark current components generated in the pixels. Since a given number of OB pixels must be read for effective OB clamping, a given period of time (OB clamp time) is required to read the required number of OB pixels.
However, in the image capturing apparatus having a plurality of read modes, when the OB pixel area is set to ensure the sufficient OB clamp time (the number of OB pixels to be read) at the highest thinning-out rate as described above, it is difficult to set a high frame rate in high-resolution video photographing. This is because when the OB pixel area is set as described above, the number of OB pixels to be read increases in reading at a low thinning-out rate, e.g., in reading all pixels. It takes an excessively long time to perform OB clamping, thereby prolonging the signal read time per frame. The same problem also occurs when performing pixel addition such as two-pixel addition or four-pixel addition in the horizontal direction.